This disclosure relates to methods of servicing a wellbore. More specifically, it relates to methods of treating a wellbore to remove mineral scale deposits.
Natural resources such as gas, oil, and water residing in a subterranean formation or zone are usually recovered by drilling a wellbore down to the subterranean formation while circulating a drilling fluid in the wellbore. After terminating the circulation of the drilling fluid, a string of pipe, e.g., casing, is run in the wellbore. The drilling fluid is then usually circulated downward through the interior of the pipe and upward through the annulus, which is located between the exterior of the pipe and the walls of the wellbore. Next, primary cementing is typically performed whereby a cement slurry is placed in the annulus and permitted to set into a hard mass (i.e., sheath) to thereby attach the string of pipe to the walls of the wellbore and seal the annulus. Subsequent secondary cementing operations may also be performed.
During the drilling operation, a drilling fluid, also referred to as drilling mud, is conventionally circulated through the wellbore as the borehole is drilled. The presence of such a drilling mud aids in the drilling operation, for example, by removing cuttings from the wellbore, (e.g., by suspending and releasing cuttings returned to the surface), controlling pressures within the subterranean formation, sealing permeable and/or semi-permeable portions of the subterranean formation, stabilizing the subterranean formation, cooling and lubricating the drilling apparatus, and facilitating completion operations. At the end of the drilling operation, in order to transition to the completion operation, a wellbore cleaning operation is typically performed, as any debris such as drilling mud and metal solids left in the wellbore can have an adverse effect on several aspects of a wellbore's completion and production stages, from inhibiting the performance of downhole tools to inducing formation damage and plugging of equipment. For example, mineral scale accumulations or deposits have to be removed from the pipes and casing prior to wellbore completion.
During the production stage, scale can develop in formation pores near the wellbore, and it can reduce the porosity and permeability of the formation. Whenever the wellbore produces water, or when water injection is used to enhance the recovery of the natural resource, there is always the possibility that scale will form. Scale may block flow of the natural resources by clogging perforations or forming a thick lining in the production tubing. Scale can also coat and damage wellbore equipment, such as safety valves, casings, production tubing, mandrels, pipes, separators, pumps, etc.
If the scale is not removed, the wellbore capacity production can diminish drastically, and in some cases scale deposits can cause the wellbore production to be shut down for a period of time. As such, scale deposits may be removed to allow the unimpeded flow of natural resources through the wellbore. Some scales, such as calcium carbonate scales for example, can be removed by dissolution with acids, which can have corrosive effects on the wellbore equipment and may damage the formation. Other scales, such as barium sulfate scales for example, however, are resistant to conventional acid dissolution treatments. Scale removal by acids, when effective, may sometimes lead to the re-precipitation or re-deposition of the scale on surfaces. One way to circumvent this problem is by using chelating agents to chelate the metal ions from the mineral scale deposits. A variety of chelating agents are commonly used for scale removal in wellbore operation procedures, however, these conventional chelating agents have very low biodegradability and tend to accumulate in the environment. Thus, an ongoing need exists for more effective compositions and methods of removing mineral scale deposits in subterranean formations and/or equipment associated with the wellbore.